An example of such composite sensor for use with a door (hereinafter referred to as composite door sensor) is disclosed in a catalogue of composite sensors available from B.E.A. Inc., entitled “ACTIV8.3”. The composite door sensor disclosed in the catalogue includes a microwave transmitter-receiver unit and an infrared emitter-receiver unit in a single casing. A microwave is used to detect an object, e.g. a moving object or pedestrian moving toward a door. When a moving object is detected by the microwave, the door is opened. Infrared light is used to detect a moving object standing stationary in the vicinity of the door. As long as the object is being detected by the infrared light, the door is kept open. Thus, an accident of a moving object being caught in the door can be avoided, and the safety of the moving object can be secured.
Infrared light used in such composite sensor for a door system tends to be adversely affected by disturbances, such as rain and snow. Infrared light is reflected not only by human bodies but also by rain and snow. Therefore a prior art composite door sensor like the one described before would erroneously detect rainfall, snowfall, puddle after the rain, or snow on the ground as an object to be detected by the sensor (hereinafter sometimes referred to as relevant object), such as a pedestrian. This causes an erroneous operation of an automatic door to open the door in spite of absence of any relevant object.
An object of the present invention is to provide a composite sensor for a door system with reduced possibility of erroneous operation of the automatic door which would be caused by disturbances, such as rain and snow.